/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 * <p>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p>
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.*;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * The <code>ShutdownHookManager</code> enables running shutdownHook
 * in a deterministic order, higher priority first.
 * <p/>
 * The JVM runs ShutdownHooks in a non-deterministic order or in parallel.
 * This class registers a single JVM shutdownHook and run all the
 * shutdownHooks registered to it (to this class) in order based on their
 * priority.
 */
public class ShutdownHookManager {
    private static Logger log = LoggerFactory.getLogger(ShutdownHookManager.class);

    private static final ShutdownHookManager MGR = new ShutdownHookManager();


    static {
        Runtime.getRuntime().addShutdownHook(
                new Thread() {
                    @Override
                    public void run() {
                        MGR.shutdownInProgress.set(true);
                        for (Runnable hook : MGR.getShutdownHooksInOrder()) {
                            try {
                                hook.run();
                            } catch (Throwable ex) {
                                log.warn("ShutdownHook '" + hook.getClass().getSimpleName() + "' failed, " + ex.toString(), ex);
                            }
                        }
                    }
                }
        );
    }

    /**
     * Return <code>ShutdownHookManager</code> singleton.
     *
     * @return <code>ShutdownHookManager</code> singleton.
     */
    public static ShutdownHookManager get() {
        return MGR;
    }

    /**
     * Private structure to store ShutdownHook and its priority.
     */
    private static class HookEntry {
        Runnable hook;
        int priority;

        public HookEntry(Runnable hook, int priority) {
            this.hook = hook;
            this.priority = priority;
        }

        @Override
        public int hashCode() {
            return hook.hashCode();
        }

        @Override
        public boolean equals(Object obj) {
            boolean eq = false;
            if (obj != null) {
                if (obj instanceof HookEntry) {
                    eq = (hook == ((HookEntry) obj).hook);
                }
            }
            return eq;
        }

    }

    private Set<HookEntry> hooks =
            Collections.synchronizedSet(new HashSet<HookEntry>());

    private AtomicBoolean shutdownInProgress = new AtomicBoolean(false);

    //private to constructor to ensure singularity
    private ShutdownHookManager() {
    }

    /**
     * Returns the list of shutdownHooks in order of execution,
     * Highest priority first.
     *
     * @return the list of shutdownHooks in order of execution.
     */
    List<Runnable> getShutdownHooksInOrder() {
        List<HookEntry> list;
        synchronized (MGR.hooks) {
            list = new ArrayList<HookEntry>(MGR.hooks);
        }
        Collections.sort(list, new Comparator<HookEntry>() {

            //reversing comparison so highest priority hooks are first
            @Override
            public int compare(HookEntry o1, HookEntry o2) {
                return o2.priority - o1.priority;
            }
        });
        List<Runnable> ordered = new ArrayList<Runnable>();
        for (HookEntry entry : list) {
            ordered.add(entry.hook);
        }
        return ordered;
    }

    /**
     * Adds a shutdownHook with a priority, the higher the priority
     * the earlier will run. ShutdownHooks with same priority run
     * in a non-deterministic order.
     *
     * @param shutdownHook shutdownHook <code>Runnable</code>
     * @param priority priority of the shutdownHook.
     */
    public void addShutdownHook(Runnable shutdownHook, int priority) {
        if (shutdownHook == null) {
            throw new IllegalArgumentException("shutdownHook cannot be NULL");
        }
        if (shutdownInProgress.get()) {
            throw new IllegalStateException("Shutdown in progress, cannot add a shutdownHook");
        }
        hooks.add(new HookEntry(shutdownHook, priority));
    }

    /**
     * Removes a shutdownHook.
     *
     * @param shutdownHook shutdownHook to remove.
     * @return TRUE if the shutdownHook was registered and removed,
     * FALSE otherwise.
     */
    public boolean removeShutdownHook(Runnable shutdownHook) {
        if (shutdownInProgress.get()) {
            throw new IllegalStateException("Shutdown in progress, cannot remove a shutdownHook");
        }
        return hooks.remove(new HookEntry(shutdownHook, 0));
    }

    /**
     * Indicates if a shutdownHook is registered or not.
     *
     * @param shutdownHook shutdownHook to check if registered.
     * @return TRUE/FALSE depending if the shutdownHook is is registered.
     */
    public boolean hasShutdownHook(Runnable shutdownHook) {
        return hooks.contains(new HookEntry(shutdownHook, 0));
    }

    /**
     * Indicates if shutdown is in progress or not.
     *
     * @return TRUE if the shutdown is in progress, otherwise FALSE.
     */
    public boolean isShutdownInProgress() {
        return shutdownInProgress.get();
    }

}
